The up conversion of digital baseband signals to RF frequency bands and the subsequent down conversion of these signals back to baseband cleanly and efficiently has always been the desired end product of modulation and demodulation systems.
Generally, traditional modulation/demodulation systems fall into 3 broad categories. The first are systems that are completely analog which incorporate a wide variety of passive and active mixers. Components in systems such as these range from passive diode and resistive mixers and summers to active, analog Gilbert cells. These systems require massive amounts of filtering (hardware intensive), are generally not power efficient, and are sensitive to noise and intermodulation distortion.
The second broad category of modulation/demodulation systems utilize low speed DACs and analog mixers for up conversion of digital baseband signals to RF frequency bands and subsequently utilize mixers and ADCs for down conversion of these signals back to baseband for digital processing. Although this system solution allows for digital control and processing of baseband signals, it still suffers from the same hardware, power, and distortion problems as the first category.
The third broad category of modulation/demodulation systems utilize high speed digital processing, high speed DACS, and high speed ADCs to directly convert digitally generated data to RF signals in transmitters and directly digitize incoming RF in receivers. Although this approach eliminates much of the hardware and resulting intermodulation distortion associated with the previously mentioned solutions, it introduces additional problems in that high speed DACs and ADCs are hard to design, expensive to fabricate, and the high speed circuitry composing these devices is power expensive.
All three presently employed modulation/demodulation methodologies are expensive in terms of power, hardware, and complexity.
The more hardware and power efficient modulation/demodulation systems can become, more digital functions can be incorporated into designs, and ultimately a wider diversity of products may be realized. With the advent and popularity of baseband digital signal processing in modulation/demodulation systems, the usefulness of circuit designs and methods that can directly digitally up convert digital data to RF frequency bands and easily demodulate incoming RF back to baseband for analog to digital conversion becomes clear.